Expose build settings to fbs_runtime

A common thing to do is to display the version of your app in the GUI. In previous versions of fbs, this was difficult. The reason for this is that the version was stored in (usually) the base.json settings file, which is theoretically available when you run from source, but not when running the frozen form of your app.

This release of fbs adds a property called public_settings to ApplicationContext. It lets you easily access certain settings (such as version) from your application. For an example, please see the associated documentation.